Negative impedance circuit



pil 1, 1941. R Q MA1-HES l 2,236,690

NEGATIVE IMPEDANCE CIRCUIT Filed Maron 5, '193s j /5J E O(/7 4 /NI/ENTORl?. C. MATHS Patented Apr. l, 1941 NEGATIVE IIVIPEDANCE CIRCUIT RobertC'. Mathes, Maplewood, N. J., assignor to atories, Incorporated, New

Bell Telephone Labor York, N. Y., a corpora tion of New York ApplicationMarch 5, 1938, Serial No. 194,036

2 Claims. (Cl

The present invention relates to circuits for producing negativeimpedance effects.

An object of the invention is to stabilize negative impedance circuitsso as to obtain more highly stable negative impedances.

A difficulty in the use of negative resistances or negative impedancesis that any normal instabilities and any normal variations of magnitudewith frequency are multiplied in their effects. Thus, in a singlenegative resistance circuit of a positive resistance R and negativeresistance -r in series,

E R-r Where I is the current flowing as a result of application of adriving voltage E. If r is about l per cent different from R in absolutemagnitude, a l per cent variation in r or in R becomes about a per centvariation in I.

Negative impedance elements are inherently less stable than -positiveresistance and irnpedance elements. It becomes important, therefore, tobe able to improve the stability of negative impedances where possible.

One method of producing negative impedance is'by positive regenerationof amplifier circuits. This may be done with series, shunt or withhybrid coil connection of input and output circuits. One example of suchmethod of producing negative impedance is found in my prior Patent1,779,382, October 21, 1930'.

In negative impedance circuits involving am-v plification with positiveregeneration, the stability is improved in accordance with thisinvention by applying a stabilizing negative feedback to a suitableportion of the system including the amplifier, thereby improving thestability of the negative impedance that is being produced.

In the drawing to which reference will now be made for a full disclosureof preferred embodiments of the invention- Fig. 1 is a schematic circuitdiagram of a negative impedance system with series positive regenerationand with a stabilizing negative feedback; and

Fig. 2 is a similar diagram of a negative impedance system usingparallel positive regeneration.

In Fig. 1 a three-stage amplifier comprises tubes I, 2 and 3 withsuitable interstagecircuits shown as the conventionalresistance-capacity type. This amplifier is provided with a negativefeedback connection I3 from its output to its input side, theconnections being made by The feedback path is means of an output bridgeand an input bridge. The input bridge 'includes resistances 5, 6 and 1,the feedback connection being madeat the junction of 5 andi as shown. Inthis way the feedback is kept independent of variations in resistanceacross the input diagonal extending from the grid to the junction ofresistances Ii and I. Similarly, the output bridge consists of,resistances 8', Il and I'II, the feedback connection being made at thevjunction of resistances 8 and 9.

thus conjugate to the output circuit across the of tube 3 and thejunction of resistances 9 and Iii. Space current for all three tubes issupplied from a suitable source such as battery I2. Condenser II is ablockingcondenser.

The feedback action and the manner in which it is attained including theinput and output bridges and their design are in accordance wi-th theteachings of H. S.,Black Patent 2,102,671, issued December 21, 1937, towhich reference may be made for a fuller disclosure.

Positive feedback is used by way of transformer I4 and circuit I5, I6,in connection with a positive impedance Z at 4 and a source of drivingvoltage E to develop a negative impedance Zn looking into the circuit atterminals I'I and I8 in the general manner disclosed in my prior Patent1,779,382 referred to above. The impedance Z is a positive externalimpedance to be neutralized in part by the negative impedance of thecircuit. The voltage drop across Z is applied to the grid of the firsttube I, is amplified in stages I, 2 and 3 and is fed back in amplifiedform through transformer I4 in aiding phase with the voltage E, whereby,with proper adjustment and proportioning, a negative impedance Zn isdeveloped between the terminals I1, I3, having a component which is thenegative of the positive impedance Z. Transformer I4 aids in producingthe required phase relation between the fed back Voltage and the voltageE.

As developed in the mathematical discussion in my prior patent supra,the negative impedance Zn is produced as the difference betweenrelatively large quantities one of which involves as a factor theVoltage amplification factor of the amplifier. The other quantities mayin any given system be simply positive resistances or reactances whichmay be constructed or controlled to be electrically very stable. Asnoted above, however, a relatively small instability in one of therelatively large quantities whose difference determines the negativeimpedance represents a large percentage instability in the diagonalbetween the plate negative impedance. Consequently, in accordance withthis invention the stability of the negative impedance is improved bystabilizing the amplier, which is likely to be the greatest source ofinstability because it includes such elements as tubes and voltagesources. The improvement in stability is substantially proportional tothe gain reduction due to the negative feedback. By providing largeexcess gain in the amplier and reducing the gain by feedback to therequired value, great improvement in stability is secured.

In Fig. 2 the parts corresponding to the elements in Fig. 1 arecorrespondingly designated and it will be seen that Fig. 2 shows theshun-t type of negative impedance circuit in that the circuit l5, I6from feedback transformer I4 is connected in parallel across positiveimpedance 4 and the series combination of source of voltage E andpositive impedance Z'. Otherwise the construction and operation of thiscircuit are the same as in the case of Fig. 1.

The invention is not to be construed as limited to the particularcircuits that have been disclosed since the details may be varied widelyin actual practice. The scope of the invention is defined by the claims.

What is claimed is:

1. In combination, a pair of terminals between which the negative of agiven positive impedance is to be produced, a source of voltage and saidpositive impedance connected in circuit with said terminals, anamplifier connected to amplify the voltage developed in said positiveimpedance and to apply the ampliiied voltage to said circuit in phasewith the voltage from said source, whereby the negative of said positiveimpedance is developed across said terminals, means to stabilize saidnegative impedance comprising a gain stabilizing negative feedbackcircuit for said amplifier and means rendering said feedback circuitconjugate with respect to the terminals of said positive impedance.

2. In a negative impedance circuit, a circuit having two terminalsacross which a negative impedance effect is to be produced, a positiveimpedance in circuit with said terminals, means developing a drop ofpotential across said positive impedance, an amplifier having input andoutput terminals, means to apply said drop of potential to the input ofsaid amplifier, said amplier having a positive feedback circuit and anegative feedback circuit, said positive feedback circuit feeding back aportion of the amplified voltage from the output to the input side inphase with said drop of potential whereby the negative impedance effectis produced and said negative feedback circuit applying a portion of theamplied voltage from the output to the input side of said amplifier insuflcient amount to reduce the gain of the circuit and stabilize saidnegative impedance effect.

ROBERT C. MATHES.

